ABSTRACT Atherosclerosis is the main cause of death and disability in industrialized nations, including the United States. Despite of significant advances in recent years, the treatment of cardiovascular disease is unsatisfied partly due to the limitations in our understanding of the atherosclerosis pathophysiology. Growing evidence indicates that atherosclerosis is a chronic inflammatory process that accelerates in the context of dyslipidemia. Gaining new knowledge on inflammatory mechanisms in this disorder is a very important endeavor as this could lead to new therapies for the treatment of this condition. Both our previous work and preliminary study indicate that dipeptidyl peptidase-4 (DPP4) plays a critical role in regulating T cell-mediated inflammation in cardiometabolic disease such as atherosclerosis and diabetes. Single cell transcriptomic analysis indicates that DPP4 was significantly upregulated in major T cell subsets from patients with atherosclerosis. Ldlr-/- chimeric mice with Dpp4-/- bone marrow showed reduced atherosclerotic plaque burden and less T cell infiltration in the plaque. Subsequent gene array and T cell functional studies suggest microtubule-associated E3 ubiquitin ligase midline-1 (Mid1) mediates DPP4-induced T cell activation and migration. Both Dpp4-/- T cells and Mid1-/- T cells showed reduced migratory activity, while lentivirus-mediated over-expression of Mid1 restored the migratory ability of Dpp4-/- T cells. Our overall hypothesis is that DPP4 activation by ligand binding regulates microtubule-mediated cytoskeleton rearrangement and endocytosis via Mid1-dependent mechanisms, resulting in enhanced T cell migration to aortic plaque, a key event in the pathogenesis of atherosclerosis. In this application, we will test the involvement of DPP4 in T cell inflammation and human atherosclerosis (aim 1). We will also use several unique animal models (including hDpp4KI, Mid1-/-, Dpp4-/-, Dpp4-/-/Mid1-/- double knockout, and multiple reporter mice) and state-of-the-art technologies (such as imaging flow cytometry and intravital 2- photon microscopy) to dissect the role of DPP4-Mid1 axis in regulating T cell activation and migration to atherosclerotic plaque (aim 2). The in vivo effects of DPP4 and Mid1 on atherosclerosis progression will be investigated in aim 3. This proposal not only addresses important knowledge gaps in DPP4 biology, but would also identify novel contributors to human atherosclerosis and other disease states where inflammation plays a critical role.